1. Field of the Invention
The present invention relates to a trench gate semiconductor device. In more detail, the present invention relates to a technology that alleviates an electric field applied to a gate oxide film that covers a trench gate electrode.
2. Description of Related Art
Japanese Patent Application Publication No. 2005-116822 (JP 2005-116822 A) discloses a trench gate semiconductor device (power MOS) that controls electric continuity between a drain and a source by voltage application to a gate electrode. The semiconductor device includes a gate trench that penetrates through a body layer from an upper surface of a semiconductor substrate and extends to an inside of a drift layer. A floating region is formed on a bottom part of the gate trench, and the floating region is separated from the body layer by the drift layer. In the semiconductor device, in a state in which a voltage by which a drain side becomes a high voltage to a source side is applied between the drain and the source (that is, in a state in which a forward bias voltage is applied to the semiconductor device), a current flows between the drain and the source (so-called on-state) when a voltage equal to or more than a predetermined threshold value is applied to the gate electrode. On the other hand, in a state in which the forward bias voltage is applied to the semiconductor device, the current that flows between the drain and the source is cut off (so-called, off-state) when a gate electrode voltage is made less than the predetermined threshold value. In the off-state, a depletion layer extends at a pn junction location of the drift layer and the body layer and extends at the pn junction location of the floating region and the drift layer, and the two pn junction locations become peaks of electric field intensity. Thus, peak values of the electric field intensity are dispersed, and a withstand voltage can be improved thereby.
In recent years, from the viewpoint of improvement in an energy efficiency and the like, an effort for achieving a higher withstand voltage of a semiconductor device is forwarded, and also a voltage applied to the semiconductor device becomes higher. In the trench gate semiconductor device, when the voltage applied to the semiconductor device becomes a high voltage, also an electric field applied to the gate oxide film becomes high. In particular, an electric field concentration tends to occur at a corner part of a lower end of the trench gate electrode, and thus the electric field applied to the gate oxide film in this part becomes high. Also in the semiconductor device according to JP 2005-116822 A, this problem still exists.